This invention relates generally to the production of hydrogen and oxygen by the electrolysis of water. In some specific embodiments, the invention is directed to the production of hydrogen by the photoelectrolysis of water, using solar radiation.
The conversion of sunlight and water into a clean, high-efficiency chemical fuel has been of great interest for a number of years. The need for such technology is more urgent as the damaging effects of burning fossil fuels has become apparent. The photolysis of water to yield hydrogen and oxygen has been demonstrated, via the visible light illumination of Group II-sulfides and selenides, such as CdS, CdSe, ZnS, and the like. Water has also been photolyzed under ultraviolet light illumination, using compounds such as TiO2, BaTiO3, and ZnO.
Photoelectrolytic devices are in many ways similar to photovoltaic devices, which include a p-n junction. In the case of photoelectrolytic devices, the p-n junction is usually replaced by a p-electrolyte-n junction (or metal-electrolyte-n junction). Electron-hole pairs are generated by the absorption of light in the semiconductor electrodes. (The semiconductor electrodes can be thought of as “photocatalysts”). The electron-hole pairs are separated within the photocatalyst, and are injected at the respective electrodes to produce electrochemical oxidation and reduction reactions. In the case of an n-type electrode, holes combine with water molecules (H2O) to produce an anodic oxidation reaction. The reverse reaction occurs at a p-type (or metal electrode), where electrons combine with protons (H+), to produce a cathodic reduction reaction. The net effect is a flow of electrons from the anode to the cathode, resulting in reduction at the latter (hydrogen formation), and oxidation at the former (oxygen formation).
Clearly, photoelectrolysis has the potential to provide an inexpensive source of hydrogen, while also providing a way to efficiently store energy obtained from solar energy conversion. However, photoelectrolytic processes continue to have serious drawbacks. For example, the processes can be relatively slow and inefficient. The poor efficiency is due in large part to the bandgap characteristic of the photocatalyst(s) employed in the photoelectrolytic cells. (“Bandgap energy” can be defined as the difference between the reduction potential and the oxygen potential of the cell). Using related terminology, the “bandgap” is considered to be the amount of energy required to promote an electron, within its orbital configuration, from the valence band to the conduction band. Consequently, solar radiation which has an energy value less than the bandgap does not generate the electron-hole pairs required for the electrochemical reactions described above. Most known photocatalysts have bandgaps which are too large for the efficient photoelectrochemical splitting of water, i.e., only a small portion of the solar spectrum participates in photocatalysis.
Various attempts have been made to improve the efficiency of the photoelectrochemical cells. For example, semiconductors based on materials like TiO2 often incorporate an external electrical bias, i.e., in addition to the basic photon energy input. (In the case of TiO2, the alignment of the band gap with the oxidation and reduction potential of water is not ideal for some electrolysis systems). The bias can function to satisfy the energy balance for the relevant oxidation-reduction equation, e.g., by providing a better potential difference between band energies and the redox potential of water. The necessary cell reactions can therefore proceed more efficiently. However, use of a large electrical bias diminishes the advantages of using solar radiation in the first place.
Tandem photoelectrolytic cells, which usually include two or more photocatalysts, have been used in an attempt to overcome the problems of having a single, inefficient photocatalyst. Various types of tandem cells have been designed. As an example, U.S. Pat. No. 6,936,143 (Graetzel) describes a photoelectrochemical system in which one photocell is mounted behind another cell. The design of the cells involves a color-based division for absorption of the emission spectrum. One cell which absorbs blue and green portions of the spectrum generates oxygen. The second cell includes a dye-sensitized mesoporous photovoltaic film. This cell converts yellow and red light from the spectrum, reducing protons in the first cell to hydrogen.
While tandem cells may sometimes provide greater photoelectrolytic efficiency, there are drawbacks associated with them as well. For example, the cell structure can be complex, and difficult to produce. Greater complexity often leads to higher manufacturing costs. Moreover, some of the non-oxide types of tandem cells can be susceptible to photo-induced corrosion. (Photo-oxidation usually occurs on the oxygen-generating side of the cell, where non-oxide surfaces tend to become oxidized, instead of liberating oxygen. According to the overall oxidation-reduction scheme, if the oxide layer becomes thick enough, then the photo-induced generation of hydrogen will diminish, effectively shutting down the cell).
With some of these concerns in mind, improved photoelectrolytic devices would be welcome in the art. The devices should be capable of producing hydrogen efficiently and economically. Novel photocatalysts used in such devices would also be of great interest. The photocatalysts would be capable of functioning as one or more semiconductor electrodes for the device, and should be obtainable and usable at reasonable cost. More specifically, the photocatalysts should exhibit bandgap characteristics which permit very effective absorption of solar radiation and conversion to hydrogen.